Why can't I unhide objects in 2.8 that were hidden in 2.79 blend files?
When hiding an object in Blender 2.79 this is indicated by the eye icon in the outliner.

 

In Blender 2.8x the eye icon has a different meaning. It indicates that the object is hidden in the current view layer (Hide in Viewport), whereas the option in 2.79 had a global effect. Hence, when loading an old file from 2.79, it is treated as Disable in Viewport (Monitor icon) instead. The difference between the two is explained in this answer.


  The Hide in Viewport option is just applied for the current view layer, whereas Disable in Viewports is global and applies to all view layers and viewports.


In order to change the visibility for the hidden object you need to enable the setting to show the Restriction Toggles.



Afterwards you can change the visibility in the outliner and the object will appear in the viewport(s) again. The effect of the restriction toggles is documented in Blender's manual.



The same options exist for the collections as well. Additionally a collection can be excluded from the view layer by unticking the checkbox in front of its name.

More information about this topic is available on the following pages:


View Layers and Collections - An introduction to the concept in Blender 2.8x
Blender 2.8: View Layers and Collections - Release notes for Blender 2.80